This invention relates to a method and apparatus for producing a spray of fine electrically charged liquid droplets and applying the spray primarily for the purpose of suppressing smoke.
One of the principal impediments both to the fighting of fires and the escape of potential fire victims is the presence of smoke which is typically created in dense black clouds when the fire occurs in an enclosed area such as a building. The smoke quickly fills corridors, passageways, hallways and other areas in the building and provides a severe restriction on the visibility of those attempting to flee the fire and to firefighters attempting to locate and extinguish the fire. It is also well known that smoke is dangerous to persons exposed to it and that it can cause extensive property damage.
Smoke from the uncontrolled burning of a hydrocarbon fuel or other combustible material is created by a combination of thermal degradation (pyrolysis) and incompleted combustion which produces an aerosol of carbonaceous particulate matter commonly known as soot. In flames where ventilation is limited, such as those found in accidental fires in buildings, partially oxidized gases and condensed phase products are formed, and this creates dense black smoke and soot. The soot particles are initially very small (less than about 20 Angstroms in diameter), and these particles later coagulate to form chain like agglomerates or clusters. Because of the ionizing environment in the high temperature flame zone, the soot particles usually acquire a positive electrostatic charge.
We have found that if small water droplets having an opposite or negative charge are injected into the smoke, even the smaller soot particles less than 1-2 microns can be captured and removed from the aerosol cloud. A combination of the physical mechanisms of impaction, phoretic forces and electrostatic attraction cause the soot particles to collect on the droplets which subsequently settle to the floor under the influence of gravity. As a result, the smoke is suppressed and visibility is greatly improved.
In the past, electrostatically charged liquid droplets have been used for a variety of purposes, including the spraying of paint and other coatings, the application of pesticides, and in gas cleaning equipment. Recent environmental concerns have also led to investigations of the ability of charged fog to remove suspended particles that are generated by industrial processes. In the equipment that has been developed for the generation of charged fogs, the electrostatic charge is imparted to the droplets in one of three ways: by ionized field charging (also referred to as ion attachment charging), by induction charging, or by contact charging. Ionized field charging involves the use of a corona to create a flow of ions which attach to the liquid droplets as they pass through the region near the electrode. Induction charging relies on a uniform electric field around the spray to transfer or induce a charge on the droplets. Contact charging is carried out by directly connecting the atomizer to a source of high electrical potential which charges the droplets as they are formed.
The technology of electrostatic fogging devices has been at least partially adapted from that of conventional sprayers. One known device uses an off the shelf spray nozzle in combination with an induction charging system which surrounds the nozzle with a metal ring connected to a source of high potential. This device is subject to clogging of the nozzle in situations where the water supply contains a high concentration of suspended solids, and its use of induction charging results in a significant electrical charge being applied only to the droplets located on the outside of the conical spray pattern in heavy sprays. The remaining droplets are only charged by natural processes and thus are unable to make best use of electrostatic attraction which is critical to effective capture of particulate matter.
Another known device, described in pending patent application U.S. Ser. No. 341,861, filed on Jan. 22, 1982, uses an electric motor, a fan, and contact charging to create a spray of electrified water droplets intended for use in the suppression of industrial emissions. Although this type of electrostatic fogger functions in a generally satisfactory manner for its intended purpose, it is not suited for use in suppressing smoke which is generated by accidental fires. The presence of a motor and fan in the device makes it too heavy and bulky to be conveniently transported and handled by firefighters. Moreover, two sources of electrical power are required, one to supply the AC motor and another to supply the high voltage DC that is required to charge the water. Another problem is that a single electrode in the water line is provided, and it serves essentially as a point source which is inefficient in applying a high electrical charge to the water.